1. Field of the Invention
The invention relates to a method and a device for operating electric arc melting furnaces and/or resistance melting furnaces, comprising a melting vessel for receiving the molten mass, whose lid and upper lateral wall are cooled by a cooling medium, preferably water, up to or inclusive of the area of the slag zone.
2. Description of the Related Art
Such cooled furnaces are known in many modifications. In these known furnaces, the furnace bottom is the only area that is not cooled and has the tendency to undergo increased wear of the refractory lining and require increased repair expenditure of the construction elements.
In order to cool at least that part of the furnace bottom in which the bottom electrodes are located, it is known from EP 02 03 301 B1 to arrange in this area of the furnace bottom at a spacing a plate through which the necks of the electrodes or contact pins are guided and to blow air into the intermediate space between this plate and the furnace bottom. With this measure the bottom electrode is cooled during the melting and tapping operation wherein, for extended operational downtimes, the cooling efficiency can be adjusted, by reducing it, such that the rate of temperature change of the bottom electrode, in particular, at the beginning or the end of the operating downtime, does not surpass predetermined maximum values.
Based on this known prior art, it is the object of the invention to provide a method for operating electric arc melting furnaces and resistance melting furnaces with which the disadvantage of only a partial cooling can be prevented.
The above object is solved for electric arc melting furnaces and resistance melting furnaces of the described kind by a shell-shaped cooling device enclosing the lower part of the melting vessel and formed as a mantle corresponding to the contour of the outer furnace wall and arranged on the melting vessel at a spacing thereto, wherein at least one inlet opening and at least one outlet opening for the cooling medium are arranged on the cooling device.
By the measure of the invention to also cool the lower area of the furnace, the furnace bottom and the lower part of the lateral walls, a more beneficial effect is achieved as a whole in regard to the service life of the refractory lining as well as of the additional construction elements of the furnace. Moreover, with the measure the invention an advantageous cooling action is also exerted onto the bottom electrode.
The cooling according to the invention is realized by means of a shell-shaped cooling device, enclosing the area of the lower furnace to be cooled, through which the cooling medium flows. The cooling medium can be a gaseous material, for example, air, or a liquid material, for example, water.
For maintaining flow of the cooling medium within the cooling device, convection can be used in the simplest case wherein, in the case of air cooling, the convection can be enhanced by a chimney which is connected with the outflow opening of the cooling device. With this chimney, it is advantageously also prevented that flames can enter the cooling device during tapping of the furnace.
Should convection not be sufficient, according to the invention it is also possible to convey the cooling medium through the cooling device by means of a conveying device, for example, a pump or a blower, arranged externally to the cooling device. Particularly for liquid cooling media, it is beneficial to convey the cooling medium in a closed circuit through the cooling device. In this connection, the cooling medium which has been heated can be cooled advantageously such that a heat recovery is possible.
The flow speed and the temperature of the cooling medium determine the cooling efficiency of the cooling device so that, according to an advantageous embodiment of the invention, the cooling efficiency can be matched to the operating temperature of the furnace by changing these parameters by means of a measuring and control device.
The cooling device which encloses the lower part of the furnace like a shell is formed according to the invention in a simple way. By means of a sheet metal, which is shaped according to the furnace contour and is arranged on the furnace at a spacing thereto, a mantle-shaped hollow space is provided through which the cooling medium flows. The hollow space has at least one inlet opening and at least one outlet opening for the cooling medium, wherein in the case of convection the inlet opening is to be expediently arranged centrally at the furnace bottom and the outlet opening laterally at the top on the sidewalls. For a forced flow by means of a conveying device, the inlet and outlet openings can be arranged differently.
For improving the cooling action by means of the cooling medium, cooling ribs, which are fastened on the furnace wall, for example, by welding, are arranged according to an advantageous embodiment of the invention within the hollow space of the cooling device. These cooling ribs are configured such that they ensure an optimal cooling efficiency without, however, substantially increasing the flow resistance of the cooling device, for which purpose they are expediently curved in the flow direction.
In order to realize the possibility of heat recovery for cooling in a closed circuit, a heat recovery device is arranged in the cooling circuit lines in addition to the conveying device for maintaining the circulation, in which the heated cooling medium can be cooled and which uses the heat released thereby, for example, by storing it.
According to one embodiment of the invention, a measuring and control system, into which the measured values of the operating temperatures of the furnace are entered, is connected with this heat recovery device and with the conveying device in order to be able to affect the temperature and the quantity of the cooling medium flowing into the cooling device.